Solar cells are known as photovoltaic devices that convert the energy from sunlight into electrical energy (for example, see PTL 1). In recent years, with the aim of improving the conversion efficiency of solar cells, much research and development has been conducted of solar cells having a heterostructure that uses a narrow-gap material such as germanium (Ge) or a silicon (Si)—Ge-tin (Sn) alloy. Ge has a band gap of 0.66 eV, and this is smaller than the band gap (1.1 eV) of the crystalline Si used in conventional photovoltaic layers. By using this type of narrow-gap material, the absorption of longer wavelength light, which until now has not been able to be utilized, can be increased, enabling further improvement in the conversion efficiency of the solar cell.